This invention relates generally to gravity flow storage silos, vessels, bins and the like, for bulk particulate solids. More particularly, the invention relates to silos used in material handling processes that require a uniform residence time of material in a silo. Examples of such processes include purging of volatiles from bulk solid particles, and curing of particles from one bulk state to another state.
A principal object of the invention is to provide a gravity flow silo that achieves uniform residence time of the particles in the sense that if a thin horizontal layer of bulk solid particles is placed on the top free surface of material in the silo and discharged, all of the particles that constituted such layer will exit the silo at substantially the same time.
A second object of the invention is to provide a silo producing relative interparticle motion throughout its volume for reasons which will be evident from the following description.
A third object of the invention is to provide a silo causing mass flow of the solids. By mass flow it is meant that all of the material is in motion whenever any material is withdrawn from the silo. With mass flow, material from the periphery as well as the center moves toward the outlet. Advantages of mass flow include the achievement of a first-in/first-out flow sequence, the elimination of stagnant, nonmoving material, the reduction of sifting segregation, the provision of a steady discharge with a consistent bulk density, and a flow that is uniform and well controlled.
Mass flow is distinguished from funnel flow, wherein an active flow channel forms above the outlet of the silo, with non-flowing material at the periphery. As the level of material in the silo decreases, layers of non-flowing material may or may not slide into the flowing channel which can result in the formation of stable ratholes. In addition, funnel flow can cause product caking, can provide a first-in/last-out flow sequence, and can increase the extent to which sifting segregation occurs in the discharging material.
A typical prior art silo for use where a uniform residence time is required is a vertical cylinder to which a converging hopper is affixed. It is known that within a tall, slender, vertical cylinder, except for localized conditions at the bottom caused by the hopper, all particles move at a constant velocity with no relative interparticle motion. This is characterized as rigid body motion. However, without relative interparticle motion some bulk solids form a stable cohesive arch at the transition, that is, the intersection between the cylinder and the hopper sections. Gravity flow of the solids then ceases. The formation of such cohesive arches at the intersection is often due to strong bonds that form between individual bulk material particles as they flow down through the cylinder section. If these bonds develop sufficient strength in the time required to flow through the cylinder, then the material may arch at the transition. Whether the bulk solid particles form sufficient strength to produce an arch is a function of the material in question and the residence time of the material in the cylinder. Increasing the size (diameter or width) of the cylinder section to reduce the propensity of the material to arch may not be possible in many cases, particularly those in which there is a requirement for a uniform residence time of all particles within the silo.
Soviet Union Patent No. 628273 issued to Vladimir Polytechnic Institute describes a silo (with inserts) for dry, free flowing materials. The inserts are comprised of converging and diverging pyramids. Vertical side walls enclose the pyramids. Both the converging and diverging surfaces have slits cut into them, with the diverging pyramid having square openings cut into the flat walls at the base of the section. These slits and square openings allow the free flowing bulk solid to pass from within the converging and diverging sections, into the outer region formed by the vertical silo walls and the pyramids. The material is also allowed to flow from the outer region, back into the pyramid sections. If the material is cohesive in nature (i.e., not free flowing), the material is likely to form arches over the slits and square openings, thus rendering gravity flow through the walls of the pyramids impossible.
By its very nature, the design with square openings at the base of the diverging pyramid sections will create dead zones of material. This implies that mass flow of material within the silo will not result. Furthermore, the design will create large velocity differentials on a horizontal slice of material (these differentials will also vary as a function of level within the silo), thereby resulting in a non-uniform residence time.